Selections (marching ants) are barely visible on the monitor, especially on high res (retina) monitors. The marching ants are so hard to see, editing the selection is just about impossible. I'd love to see a setting in preferences where the marching ants could be made more visible.

Are you running Photoshop CC 2019 (v 20.x). There is change in the most current version that increased the width of the marquee drawing to match the UI Scale factor. On windows, it's it's 1px per 100% scale, so 200% is 2px wide, 300%is 3px wide, and 400% is 4 px wide, etc. On Mac, it's 1px at 100% scale and 2px at retina (200%)